A common component of a Heating, Ventilation, and Air Conditioning (HVAC) system is an economizer. The economizer is an arrangement of dampers, actuators, sensors, and controllers that are used to bring outside air into the space. It serves two primary purposes by providing ventilated fresh air to meet the needs of occupants and free cooling benefit when conditions are appropriate.
There are various economizer control strategies relying on different sensor types (dry bulb, enthalpy, and dew point) that are used to determine when outside air is appropriate for free cooling. When the thermostat, or other controls, communicate a cooling call to the HVAC unit, the economizer controller uses one of the strategies above to determine if the outside air is suitable for free cooling. If the air is suitable, the outside air damper will open, and the system will use outside air to address the cooling needs of the space. The current practice of economizer use waits until there is a cooling call before the economizer cycle is engaged. It is typically used as the first step in responding to a call for cooling from the HVAC system. A secondary step is to initiate costly mechanical cooling. There is no electrical cost to use the economizer cycle so it is considered free cooling. The present invention expands the use of the economizer cycle beyond the current practice of waiting for a call for cooling.
Building codes have established minimum fresh air requirements to provide proper ventilation to an occupied space. Setting a minimum economizer outside air damper position is the most common way that the fresh air is introduced to the building via the HVAC supply fan. HVAC systems are designed to meet the heating and cooling needs of a facility based on the hottest and coldest conditions. Similarly, the ventilation rates are based around the maximum number of occupants the space is likely to experience. In most cases this leads to an over-ventilation of the space. In the late 1990's a control practice called demand control ventilation (DCV) was introduced that allows economizers to set a minimum base ventilation level based on square footage and increase ventilation based on an increase in occupancy as measured by a sensor such as a CO2 sensor. DCV saves energy by addressing the over-ventilation that is present in a standard system. The current art surrounding Demand Control Ventilation does not take into account outside air temperature or the heating/cooling needs of the space. The invention is an improvement over the current art of DCV in that it addresses the unintended consequence of Demand Control Ventilation as it is currently practiced. The inventors have documented that current DCV approaches result in increased energy use under certain common conditions. A decrease in the minimum outside air damper position, which is inherent in the current practice of DCV, actually deprives the treated space of outside air that is beneficial. This occurs when the treated space has a cooling disposition and the outside air is suitable for free cooling benefit. Commercial buildings have a consistent increase in what is known as cooling load over the course of a typical day. Once a facility is occupied in the morning, the internal loads increase as the number of occupants increase, computers and machinery are activated, lights are turned on, and external temperatures and solar impact increase. Cumulatively, these factors cause the internal temperature within the occupied space to increase as the day goes on. It is this increase in the space temperature above the heating setpoint that constitutes a cooling disposition. Eventually the temperature will increase to a point that the HVAC cooling setpoint will be exceeded, causing the cooling cycle to be initiated. The first time this occurs in any given day is considered the first cooling call of the day.
The current practice of DCV reduces the percentage of outside air delivered to the space regardless of the disposition of the space. Reducing the amount of beneficial outside air when the space has a cooling disposition increases the rate of temperature rise within the space and the first cooling call of the day will occur earlier than had DCV not been deployed. DCV is indeed an energy efficiency strategy when outside air is unfavorable but not when outside air is favorable for meeting the cooling needs of the space. It then has a detrimental impact on energy use. This fact has eluded the energy efficiency community. The inventors have unique resources that brought this fact to light. The invention addresses this problem in the prior art.
HVAC systems can be equipped with a constant volume or variable speed fan. The variable speed fan will modulate based on tile demands of the space that is served by the fan. When the space is in the dead-band, or satisfied state, there is the opportunity for additional energy savings. Traditional HVAC systems treat the variable fan and the economizer as separate pieces, and they are often operated independently of one another. The inventors have addressed this system deficiency, and operate the fan and economizer as a single assembly that is used to provide ventilation air to a space. The efficiency of the system is increased by slowing down the fan speed and opening up the economizer to achieve the equivalent outside airflow. The fan energy is reduced when it runs at slower fan speeds. The current invention can operate in conjunction with either a constant volume or variable speed fan system. The energy improvement over the existing art in economizer control is amplified with the presence of variable fan speed capability by delivering favorable outside air for free cooling in the comfort dead-band with only a fraction of the fan energy used by a constant volume fan.
Economizers are subject to a host of field related issues and the vast majority of the economizers don't work. Many technicians are not properly trained on how to test economizers and they are often ignored during preventative maintenance visits. Depending on the specific economizer failure it is possible that it will result in the ability to use outside air for free cooling and increase overall HVAC system energy use. These failures are such that they may cause more energy use than if the unit had never been equipped with an economizer. While required by mechanical and energy codes in most jurisdictions, the assumption is that they will operate as intended. It has been well documented that economizers not only have a high failure rate, the problems often go undetected for years. The lack of operator feedback when an economizer failure occurs is a major issue that experts are seeking to find a viable solution for.
The present invention addresses the deficiencies in the current use of economizers for free cooling and demand control ventilation while providing advances in operational accountability. By leveraging new sensors and control sequences, economizers can be operated in a smarter way that will bring a new level of energy savings and assure that they persist for the life of the system.